Time-lapse of multi-component seismic modeling of CO₂ fluid replacement in Redwater Leduc Reef, Alberta

Taher M. Sodagar, Donald C. Lawton

The Devonian Redwater reef, northeast of Edmonton, Alberta, is being evaluated for geological storage of CO₂ for the Heartland Area Redwater CO₂ Storage Project (HARP). It is located close to large sources of CO₂ in the Redwater-Fort Saskatchewan-Edmonton region. The main objective of the study was to build a 2D geological model of the Redwater reef, from the reef center to off-reef, and investigate the seismic response of the reef to CO₂ saturation in the Leduc Formation. Fluid substitution and seismic modeling were undertaken to generate PP and PS synthetic seismic data to study the consequences of CO₂ saturation on the seismic response of the various reef facies and formations below the reef, based on seismic attributes and character. Common shot ray tracing modeling was undertaken to evaluate variations in the seismic response of the Redwater reef along the 2D line across the margin of the reef for CO₂ saturation in the Upper Leduc interval. The input geological model was based on well data and depth-converted seismic data from the interpretation of legacy 2D seismic lines in the area. Seismic reflections display positive structure below the reef in time sections due to the lateral velocity change from on-reef to off-reef, but are corrected in the depth sections. Terminations and the lateral position of the Upper Leduc and Middle Leduc events are clear on the pre-stack time-migrated section and a modest improved on the depthmigrated section. Higher amplitudes at the base of Upper-Leduc member are evident near the reef margin due to the higher porosity of the foreslope facies in the reef rim compared to the tidal flat lagoonal facies within the center of the reef. Time-lapse seismic modeling predicts a reasonable amplitude difference for the seismic data before and after CO₂ saturation, particularly for reflections from the Upper Leduc, the top of the reef rim, and the Mid Leduc near the reef edge.